1. Field of the Invention
The present invention relates to a Schottky barrier rectifier, and more specifically to a Schottky barrier rectifier suitable for a power supply.
2. Description of the Prior Art
As is well known, a Schottky barrier diode is characterized by a low forward voltage and a high speed performance, and is thus suitable for supplying a large current with a low voltage. However, it is not suitable for high withstanding voltage because it involves disadvantages such as a great leakage current and others. Hence, various attempts have been made to improve its withstanding voltage characteristics by introducing therein the high withstanding voltage characteristics of a pin type diode. A typical example of a conventional Schottky barrier diode will be described with reference to FIGS. 1A to 2B.
FIG. 1A is a cross-sectional view showing a Schottky barrier diode with the simplest structure indicated for referential purposes. A barrier electrode 20, such as chromium, is disposed on the surface of an n.sup.- type epitaxial layer 2 of a wafer or chip 10 such that the barrier metal contacts the epitaxial layer 2. An electrode membrane 8 is disposed on an n.sup.+ type substrate 1 of a high impurity concentration. When the diode is allowed to conduct with the barrier electrode 20 as a positive-side terminal and the electrode membrane 8 as a negative-side terminal, its forward voltage is very low. If a reverse bias is applied, however, there will be a large leakage current as stated previously. In this diode, the n.sup.- layer 2 must have a high specific resistance and a large thickness for withstanding a high voltage, but this results a very high forward voltage. Accordingly, the conventioned Schottky barrier diode is unsuitable for a high withstanding voltage.
A conventional example as illustrated in FIG. 1B is provided with pn junctions formed by diffusion of p regions 3 to have a low leakage current characteristics when reversely biased. An n.sup.- type epitaxial layer 2 is grown on an n.sup.+ type substrate 1 of a chip 10. On the surface of the epitaxial layer 2 as a resistive semiconductor region, there are formed a plurality of p type diffusion regions 3 spaced from each other, and a barrier electrode 20 is further provided thereon. The barrier electrode 20 is conductively connected with the p type diffusion regions 3, and forms a Schottky junction with the semiconductor region 2. Since the p type diffusion regions 3 form pn junctions with the semiconductor region, by suitably setting the depth of the p type regions and the space between the adjacent p type diffusion regions 3, a depletion layer DZ is spread from the p type diffusion region 3 to the surface portion of the semiconductor region 2 upon the application of a reverse voltage. As a result, the region 2 is pinched off by the depletion layer whereby a reverse leakage current is decreased. However, when the diode illustrated in FIG. 1B is designed for high withstanding voltage the epitaxial layer 2 also should be made to have a high specific resistance and a large thickness and as a result the forward voltage becomes high and the loss in ON state increases.
FIG. 2A is a cross-sectional view of a diode of an SPiN type. With this diode, the characteristics of a pin type diode are combined with those of a Schottky barrier diode so that the above-described problems can be solved. As illustrated in the drawing, its structure is the same as that of the diode shown in FIG. 1B, except that the right-to-left dimensions of the p type diffusion region 3 are greater than in FIG. 1B. Because of this arrangement, the flow of the electron e in the n type semiconductor region 2 is markedly curved, and there is a drop in the potential at the underside of the central portion of the bottom of the p type diffusion region 3 in the semiconductor region 2. Thus, the pn junction between the p type diffusion region 3 and the semiconductor region 2 is forward biased, and the hole h is injected from the p type diffusion region 3 into the semiconductor region 2 as illustrated.
In the semiconductor region 2 where the hole h is injected, the number of carriers sharply increases owing to conductivity modulation, thereby contributing to a current. Hence, the apparent resistance of the diode, accordingly the forward voltage in the ON state, decreases. In the OFF state of the diode, the depletion layer DZ extends in the surface portion of the semiconductor region 2 lying between the p type diffusion regions 3, as in FIG. 1B. This means that the SPiN type diode of FIG. 2A is suitable for a high withstanding voltage, and can suppress the increase in the forward voltage associated with its high speed operation, by relying on the conductivity modulation effect.
As described above, it is possible to decrease the leakage current of a Schottky barrier diode by incorporating the characteristics of a pin type diode. It is also possible to maintain a balance between the operating speed and the forward voltage of the diode by utilizing a conductivity modulation effect. However, when it is made to have a high withstanding voltage characteristics a current also flows when the reverse voltage is applied due to excess carriers which don't disappear and remain in the OFF state as in the case of a pin type diode. Because of this a power loss, a switching loss, during a transient reverse recovery action performed from the ON state until the OFF state of the diode.
FIG. 2B shows the above behavior by way of the waveforms of voltage, V, and current, I, obtained when the diode of FIG. 2A changes from the ON state to the OFF state. The voltage V changes from a low ON-voltage, Vn, to a reverse voltage, Vr, via a transient voltage, Vt, which is dependent on the circuit conditions, etc. of the circuit to which the diode is connected. During this period, the depletion layer DZ widens, and a reverse recovery time, tr, is taken until a stable OFF state is reached. During this time, the current I changes from a large ON-current to zero. However, a current with the reverse direction, so-called reverse recovery current, based on charge migration flows in a transient condition persistent until the depletion layer DZ completely broadens. This current serves as a reverse current, Ir, and the time integral of the product of the reverse current Ir and the transient voltage Vt constitutes a power loss associated with the ON-OFF switching action of the diode.
This switching loss, needless to say, occurs at each ON-OFF operation of the diode, and slightly differs depending on the conditions of the circuit to which the diode is connected. However, the loss becomes enormous in proportion to the frequency of the circuit used. Hence, the applicable frequencies are limited by high frequency loss, with the high speed performance of this kind of diode not being effectively used.